The present invention relates generally to arc fault detection apparatus and methods, and more specifically to an arc fault detection apparatus and method that provides for better discrimination of electrical arcing events from nuisance loads by employing a comparator circuit with a variable threshold voltage that varies continuously with the input line voltage.
Arc fault detection apparatus are known that may be employed to provide discrimination between electrical arcing events and nuisance loads. For example, an arc fault detection apparatus may include a current sensor, an input sense circuit, an arcing sense circuit, a power supply, a tripping (firing) circuit, a processing unit, and an electromechanical interface. In a typical mode of operation, the current sensor monitors the power input via the electromechanical interface, and provides high frequency components of the power input to the input sense circuit. The input sense circuit then filters and rectifies the AC signal at its input, and provides this sensed signal to the arcing sense circuit, which, in turn, provides voltage levels indicative of possible electrical arcing to the processing unit. Next, the processing unit measures the voltage levels and analyzes the voltage measurements using one or more algorithms to determine whether the voltage levels resulted from an arc fault or a nuisance load. In the event the detected levels resulted from an arc fault, the processing unit activates the firing circuit, thereby tripping the electromechanical interface to disconnect the power input from the power output. By making a determination as to whether the AC signal sensed by the input sense circuit resulted from an electrical arc fault or a nuisance load before tripping the electromechanical interface, the susceptibility of the arc fault detection apparatus to nuisance tripping is reduced.
In a typical embodiment of the above-described arc fault detection apparatus, the arcing sense circuit includes a comparator circuit with a constant threshold voltage. The input sense circuit provides the filtered and rectified AC signal at its output to this comparator circuit, which operates to compare the level of this sensed signal with the level of the constant threshold voltage. When a significant change in the load current occurs (i.e., when a significant “di/dt event” occurs), the level of the sensed signal may exceed the constant threshold voltage level, causing the output of the comparator circuit to be driven to its rail. The comparator circuit then provides one or more voltage levels to the processing unit, which measures the voltage levels and analyzes the voltage measurements to determine whether the detected di/dt event(s) resulted from an arc fault or a nuisance load.
Although the arc fault detection apparatus described above has been successfully employed in many different applications for detecting electrical arcing, there is a need for an arc fault detection apparatus that provides for better discrimination of electrical arcing events from nuisance loads. For example, the creation and cessation of electrical arcs can cause sudden changes in the load current (i.e., sudden di/dt events) to occur near the zero crossing points of the input line voltage, typically within a specified time window centered on the zero crossing points. The Applicants have recognized, however, that nuisance loads can cause sudden di/dt events to occur both within and outside of these specified time windows. The Applicants have also recognized that some types of nuisance loads can cause a greater number of sudden di/dt events to occur outside of these specified time windows than within these time windows.
Because the arc fault detection apparatus described above employs a comparator circuit with a constant threshold voltage within its arcing sense circuit, the comparator circuit compares voltage levels corresponding to sudden di/dt events with the same constant threshold voltage level, whether or not these di/dt events occurred within or outside of the specified time windows centered on the line voltage zero crossing points. Moreover, because the levels of sudden di/dt events resulting from nuisance loads that occur outside of these specified time windows can exceed the level of the constant threshold voltage, the above-described arc fault detection apparatus may incorrectly characterize such sudden di/dt events as arc faults, thereby resulting in nuisance tripping.
It would therefore be desirable to have improved arc fault detection apparatus and methods that avoid the drawbacks of the above-described arc fault detection apparatus.